US lawmakers will likely give President Barack Obama authority to train Syrian rebels to combat jihadists, but Democrats and many Republicans are wary of repeating America's Iraq war debacle.

Mindful of the risks of war in the Middle East, Congress will rally to an embattled commander in chief's strategy to defeat the Islamic State (IS) extremist group, but seek to keep his war powers in check.

This has led to an introspective mood in Washington, where lawmakers have only a few days before hustling home to hit the campaign trail, essentially leaving the US Capitol empty until after November's congressional election.

Republican Senator Mark Kirk reflected the mood of many, saying he supports a swift vote backing Obama's effort to train and arm vetted, moderate rebel forces in Syria as a key plank in battling IS.

But, he told AFP, "my constituents are understandably concerned about Iraq 2.0."

A chief architect of the 2003 US invasion that plunged Iraq into chaos, former vice president Dick Cheney, visited Washington this week urging Obama to get "back on offense in the war on terror."

And House Republican Peter King urged the president to brace Americans for a war that could last "10 or 15 years."

House Speaker John Boehner, who habitually clashes with Obama, struck a unifying tone by urging lawmakers to "give the president what he's asking for" on counter-terrorism measures.

But skepticism and outright resistance abounds. Who better to convey the angst over potentially triggering another war than the lawmakers who now regret voting for the Iraq invasion?

In 2002, after Democratic Congressman Brad Sherman suggested various alternatives to war in Iraq, he eventually gave president George W. Bush "all the blank check authority he asked for."

Looking back, Sherman would have preferred Congress to authorize Bush's White House to use force under narrower grounds, something he and other lawmakers aim to do now.

"I would worry if we ignore ISIS, and I would worry if this leads to a war similar to the Iraq war," Sherman told AFP. "There are concerns in every direction."

Democrat Jim Moran voted against Iraq war authorization. "We've been burned by Iraq and we're not going to let that happen again," he said.

The White House argues it has authority for expanded US military intervention in Iraq and Syria under the 2001 authorization of the use of military force (AUMF) against Al-Qaeda, which Congress passed overwhelmingly after 9/11.

That AUMF was open-ended, and critics say a new one is needed to limit Obama's IS actions.

- In to win? -

"I think it's incredibly poor judgment by the administration, regardless of their believing they have the authorities already to do what they're doing, not to seek aggressively and explicitly an authorization for the use of military force," fumed Senator Bob Corker.

He is the top Republican on the Senate Foreign Relations Committee, which is drafting a "tailored AUMF" for congressional consideration.

Senate Democrat Chris Murphy said such a limited AUMF could win approval. Independent Senator Angus King said he thought the White House would not be opposed.

And moderate Republican Senator Susan Collins said she would hope to have the opportunity to "put constraints" on such an AUMF if needed.

Yet Boehner acknowledged that some Republicans are gunning for tougher action.

They include conservative Michele Bachmann, who invoked America's past battlefield glory as she pressed the case.

"We need to make a decision like we did in World War II: either we're in to win, or we're out. And that is not the strategy that the president proposed," she said.

Some Democrats, including Senators Mark Begich and Mark Udall, who are locked in tough re-election battles, and Murphy warn of dangers of equipping moderate Syrian rebels, who are accused of collaborating with extremist groups.

"We don't have the luxury of time when it comes to air strikes inside Iraq against ISIS," Murphy said. "But we have time when it comes to the question of what our policy on the Syrian civil war will be."

Congress recently has flexed its muscles to thwart war-related executive action.

It blocked the proposed transfer of Guantanamo detainees to US soil, initiated touch sanctions against Iran over its nuclear program and killed Obama's plan last year to bomb regime forces in Syria.

And in an effort to reclaim its war-making powers, the House of Representatives voted last July to require congressional authorization for any sustained presence of combat troops in Iraq.